In this longitudinal study of the development of mastery motivation, the goals are to determine whether mastery motivation in infancy is predictive of later mastery behavior and developmental competence, and to determine the role of early parent-infant interaction in the development of mastery behavior at 2 1/2 years. The relationships between cognitive functions and mastery motivation during the preschool years are also being studied. An important part of this study has been the development of methods to assess similar aspects of mastery at 2 1/2 years as were studied at 6 and 12 months of age. Data have been collected in a focused play session, an observation of the child's free play with the mastery toys, observation of the children's mastery of a learning problem, and a brief observation of mother-child interaction with another set of toys. Data on cognitive development have been obtained by the McCarthy Scales of Children's Abilities. Our analyses suggest that measures of mastery motivation in infancy do predict later competence. Furthermore, the investigation suggests that mastery in early childhood has a similar form to that observed in infancy, indicating stability in the structure of this behavior.